Lack of clean drinking water is still the primary cause for disease, suffering and ultimately death in many parts of the world. Even where water is available to the public, often times the available water is contaminated by chemicals used in agriculture, e.g., by industrial contamination or by sewage seeping into the water supply. Additionally, areas in close proximity to an ocean have a water source of high salinity, and consequently not suitable for drinking.
Water that is centrally treated is also not safe in many parts of the world as positive pressure is not maintained at all times in the distribution network for the water. Leaks in the distribution network can cause contamination of the water in this system. Furthermore, the multiple points where the water is stored after an initial treatment, e.g., storage tanks, lack any kind of continuous supervision and sanitation. In particular, storage tanks are not cleaned regularly, thereby becoming sources of contamination and having an ecosystem of their own with all sorts of insects, animals, bacterial growth, and algal growth.
The use of bottled water has grown in metropolitan cities. However, in rural areas, this is not possible, nor desired, since transportation of the bottled water to the end users is often difficult and since the indiscriminate use of plastics for the bottled water has caused a disposal and recycling nightmare.
In effort to solve such dilemmas with existing water supplies, there have been extensive efforts in the field of filtration to purify water sources. Existing technologies for filtration require the use of continuously replaceable consumables having multiple stages of filters to maintain the system in optimal state. Once these consumables are not replaced due to neglect or non-availability, the quality of the output water (otherwise referred to as product water) from these systems is severely degraded and in many cases becomes worse than the actual input water due to internal contamination.
There are two general classes of water purification technologies: one is based on the principle of evaporation and condensation, or thermal distillation, and the other is based on membrane filtration. Among membrane filtration techniques, reverse osmosis (“RO”) and electro-dialysis are the most representative. For thermal distillation, there are various vacuum thermal desalination techniques available for large, high capacity plants, as well as atmospheric distillation techniques, also called HDH, which are more suited for small purification devices.
The rapid advances of the RO based technologies in recent years have made RO the favorite among all water purification technologies owing to its low initial capital costs and high energy efficiencies. For seawater desalination, the specific energy cost of RO (when energy recovery is used) is between 4 to 7 kWh/ton of purified water, while most large thermal desalination plants which use MSF (multiple stage flash evaporation) and MED (multiple effects evaporation distillation) have specific energy expenditure between 20 to 200 kWh/ton. The HDH systems fare even worse in this respect with a specific energy cost ranging from 150 kWh/ton to more than 400 kWh/ton. The sole exception to this comparison is mechanical vapor compression (“MVC”) which can achieve a specific energy consumption level comparable to that of RO, with a range from 4 kWh/ton to just below 12 kWh/ton).
However, thermal distillation generally produces highly purified water with a TDS (total dissolved solid) level well below 1 ppm (part per million), while it would be impractical for RO plants to produce water purity of less than 20 ppm or so. RO is also unable to filter out light weight dissolved chemical molecules if their sizes are comparable to the average pore size of the RO membrane. RO is also far more prone to fouling, scaling, and plugging of the membrane, and rapid oxidation can easily destroy the membrane if it is directly exposed to air. While all water purification techniques require pretreatments or pre-filtering to reduce the probability of fouling and to ensure proper operation of the main purification process, RO typically requires more pretreatments to protect its membranes from failures, and the standard half-life of an RO membrane is about two years, hence the costs of its consumables represent a large part of its total operational cost.
The low initial cost advantage of RO lies primarily in its exceptional packing density, or area to volume ratio. While thermal distillation relies on heat exchange surfaces to reclaim latent heat in order to lower its energy cost, RO and other membrane techniques rely on large filtration surface to separate clean water from brine, hence packing density plays a very important role in both classes of purification technologies. Having a large surface area not only can increase water production, but also can reduce the surface loading factor, which is the rate of purified water production per unit surface area. Reducing surface loading can drastically improve operational efficiency at the cost of reducing water production rate since it greatly reduces internal entropy productions in both RO systems and thermal distillation plants.
Although MVC thermal distillation technology has largely caught up to RO in terms of specific energy cost, its initial capital cost is still far higher than comparable RO technologies owing to its much lower packing density. HDH systems are typically lower in costs than RO and have the potential of producing purer water than RO because of its low temperature atmospheric pressure operations. However, the extremely low specific energy efficiencies of these systems have been the main obstacles to their wide acceptance.
Another drawback is that existing distillation technologies are far too costly to implement since these technologies use a large amount of energy to convert water to steam before recondensing the saturated gases and since these technologies are typically built with expensive stainless steel or other costly metals.
One of the major disadvantages of the existing distillation techniques is the requirement to employ high strength materials for the containment and heat exchange walls. HDH partially solves the problem by using atmospheric pressure evaporation (humidification) and condensation (dehumidification) which obviates the need to utilize high strength materials and replace them with cheaper and thinner materials such as plastic substrates.
Another disadvantage of the existing distillation techniques is the comparatively low packing density, or surface to volume ratio of the heat exchange surfaces. By way of example, spiral wound filters and hollow tube RO filters have packing densities which are orders of magnitude higher and permit smaller filtration plants to be built for the same capacity. Higher packing density in the case of distillation plants could also mean lower loading on the heat exchange surfaces for the same water production capacity, which drastically improves latent heat recovery efficiency while maintaining the same water production capacity.
Still another disadvantage for some of the existing distillation techniques is the lack of direct 2-phase to 2-phase heat exchange. In order to have a direct latent heat exchange, both the evaporator and the condenser side of the heat exchange surfaces must belong to the same wall. Also, both sides of the common heat exchange wall must contain a 2-phase flow, which means both sides should have a liquid phase component and a gaseous phase component in the composite flows.
FIG. 1 illustrates a diagram of a prior art method and apparatus for water purification using HDH. The prior art comprises a vertical heat exchange wall 10 between the evaporation chamber 12 and the condensation chamber 14. Feed water 16 is sprayed downward, near the top of the evaporation chamber 12, via a sprayer 18. An air blower 20 blows against a falling mist 28 of the feed water from the bottom of the evaporation chamber 12. There is also a brine tray 22 at the bottom of the evaporation chamber 12 for storing concentrated brine 24, the remnants of the feed water that did not evaporate. The brine 24 in the brine tray 22 can be removed via a brine outlet 26 of the evaporation chamber 12. The vertical heat exchange wall 10 allows latent heat from the condensation chamber 14 to flow to the evaporation chamber 12 (see dotted arrows for that general direction). As a portion of the feed water 16 evaporates, the saturated gases are directed to the condensation chamber 14. The condensation chamber 14 then condenses the saturated gases and produces product water 30. The product water 30 is pooled and directed out of the condensation chamber via an outlet 34 for storage or use. The non-condensed gases are directed out of the condensation chamber 14 via an air outlet 32 near the bottom of the condensation chamber 14 in an open loop process. Since the latent heat exchange process does not fully recover the latent heat for reuse, an additional heat source in the form of a heater 36 is required to introduce further steam into the condensation chamber 14 and to preheat the feed water.
By placing the condensation chamber 12 side by side with the evaporation chamber 14, separated only by a common wall 10 which serves as the heat exchange wall, the latent heat generated from the condensation of the water vapor is transferred to the evaporator to heat the feed water, which eliminates one of the major drawbacks of the HDH distillation process.
Unfortunately, due to the design of the prior art, several inefficiencies are apparent. First, the vertical heat exchange wall 10 is not fully utilized since most of the latent heat transfer is inefficiently transferred from gases in the condensation chamber 14 to other gases in the evaporation chamber 12. This is due to the vertical arrangement of the heat exchange wall 10 and due to the misting of feed water 16 downwards into the evaporation chamber 12.
In a vertical heat exchange wall arrangement, filmwise condensation, first studied by Nusselt, is generally recognized as being a more efficient condensation mechanism as the latent heat released at the outer boundary of the liquid film condensate is transferred directly to the heat exchange surface without going through gases. However, in order for this to occur, the heat exchange surface must have a strong affinity to said liquid, i.e., the surface must be strongly hydrophilic. This is not the case with the prior art with its plastic heat exchange surface. The low affinity (wettability) of the plastic heat exchange surface to liquid makes it hard for the condensing liquid on the condenser side to form filmwise or dropwise condensation, and to form filmwise evaporation on the evaporator side; this drastically reduces the heat transfer efficiency and lowers the fraction of the latent heat that can be recovered.
Lower latent heat exchange performance increases internal entropy production. As will be clear below, any increase in internal entropy production decreases total system efficiency and/or reduces water production rate. Since mechanical work does not introduce additional entropy flow into the system, it is preferred over direct heat input in general cases. However, when the input heat is derived from waste heat or other low cost heat sources, it might be more preferable to use those heat sources as the input instead of mechanical work input despite the latter's more efficient utilization of the energy.
In addition, the open loop process does not reuse the sensible heat that still remains within the non-condensed gasses that are routed via the air outlet 32 from being reused. Although rerouting the non-condensed gases to the bottom of the evaporation chamber can recoup some of the waste heat, such a process is inherently inefficient owing to the large temperature difference between the non-condensed gas and the feed water. The evaporation chamber 12 also requires a large volume to generate any appreciable amount of product water owing to the relatively low surface to volume ratio of the prior art design.
Another main drawback of said prior art is its use of hot steam injection to provide the needed heat input for evaporation. As will be explained in a great deal more detail below, any direct heat input through hot fluid injection or direct heating of the system introduces a continuous stream of entropy into the system which must be ejected in order to keep the total entropy within the system finite. Such entropy ejection leads to increased energy consumption which decreases the overall system efficiency and/or production rate.
Therefore, it is desirable to present novel methods, systems and apparatuses for filtration that addresses all of the above drawbacks.